1. Field of the Invention
The present invention relates in general to an open type magnetic field generating system arranged only below a bed which should be loaded with an object to be inspected, and an MRI (Magnetic Resonance Imaging) system using the same. More particularly, the invention relates to a magnetic field generating system suitable for an interventional MRI system and a system using the same.
2. Description of the Related Art
The terms which will be used in the following description are defined as follows:    “Composite magnetic field”: A magnetic field which is obtained by composing a static magnetic field generated by means for generating a static magnetic field and a gradient magnetic field in a vertical direction.    “Nonuniformity of a static magnetic field”: A difference between maximum magnetic field strength and minimum magnetic field strength of a static magnetic field in an imaging area.    “The degree of uniformity of a static magnetic field”: A value which is obtained by dividing “nonuniformity of a static magnetic field” by a mean magnetic field strength of a static magnetic field in an imaging area.    “Nonuniformity of a composite magnetic field”: A difference between maximum magnetic field strength and minimum magnetic field strength of a composite magnetic field in an imaging area.    “The degree of uniformity of a composite magnetic field”: A value which is obtained by dividing “nonuniformity of a composite magnetic field” by mean magnetic field strength of a composite magnetic field in an imaging area.
A method which is most effective for reduction in medical cost is to shorten an in-hospital period of time. It is expected to dramatically shorten a period of time for medical treatment in a hospital due to the spread of a low invasive operation such as an operation under use of an endoscope or a low ultrasound coagulation therapy. In particular, an MRI system (e.g., JP-A-10-57344, JP-A-10-57345, and JP-A-10-57346: the prior art-1) which has a wide opening part to be able to access a patient from a multi-direction by a user (doctor) is regarded as the most powerful system for realization of the low invasive operation because there is no problem in the radiation exposure and hence it is expected to make an interventional MRI system fit for practical use. FIG. 2 is a perspective view showing an example of a conventional MRI system having a wide opening part. In the figure, there are shown static magnetic field generating means 210 arranged in a upper position and static magnetic field generating means 200 arranged in a lower position which face each other in a vertical direction.
The level of uniformity of a static magnetic field required for a conventional MRI system is equal to or lower than 10 ppm in an area of about 30 cm. On the other hand, for the purpose of acquiring MR images under the nonuniform static magnetic field, there are known a method of reducing distortion in an image utilizing a magnetic field map which is previously measured (K. Sekihara: “NMR Imaging for Magnets with Large Nonuniformities”, IEEE, TRANSACTIONS ON MEDICAL IMAGING, Vol. MI-4, No. 4, December 1985, pp. 193 to 199: the prior art-2), and a method of giving positional information by controlling dynamically an RF magnetic field (JP-A-8-322814: the prior art-3, JP-A-9-019410: the prior art-4).
Though the open degree of the conventional MRI system shown in FIG. 2 is higher than that of any of general MRI systems in which a static magnetic field is formed in a tunnel type space, it is difficult to carry a large operation tool or a system of ultrasound coagulation therapy between the static magnetic field generating means arranged in a upper position and the static magnetic field generating means arranged in a lower position, and hence it is required from a user (doctor) to further increase the open degree. If the construction is adopted in which the static magnetic field generating means 210 arranged in the upper position and shown in FIG. 2 is removed and the magnet (the static magnetic field generating means 200) arranged only in the lower position (hereinafter, referred to as “the open type magnet” for short, when applicable) is used to realize an MRI system having the higher open degree (hereinafter, referred to as “an open type MRI system” for short, when applicable), then large operation tools or a system of ultrasound coagulation therapy can be used readily.
The most important problem when the open type magnet is used is to realize the degree (equal to or smaller than 10 ppm in an area of about 30 cm in a vertical direction) of uniformity of a static magnetic field having the level required for the conventional MRI system. In the open type magnet using only the lower magnet placed on a floor, the degree of uniformity of a static magnetic field becomes about 10% even in an area of 20 cm in a vertical direction, which leads to the degradation the degree of which is about 10,000 times (four figures) as large as that of the existing MRI system.
In general, while if there is the nonuniformity in distribution of a static magnetic field, this leads to the distortion or blurring of an MR image, or a lack of a signal, when the uniformity of a static magnetic field is degraded so as to become about 10,000 times as large as that of the existing MRI system, it is difficult to acquire a nuclear magnetic resonance signal itself. If the methods of the prior arts-2, -3 and -4 are utilized, then even when the degree of uniformity of a static magnetic field is degraded by equal to or larger than one figure as compared with the existing MRI system, it is possible to acquire an image having less distortion. However, when the degree of uniformity of a static magnetic field is degraded by four figures as compared with the existing MRI system, it is difficult to apply any of the methods of the prior arts-2, -3 and -4.
The correction technique for reducing the image distortion utilizing the magnetic field map shown in the prior art-2 is the correction technique which is established on the assumption that the strength of a read-out gradient magnetic field is much larger that that of nonuniformity of a static magnetic field. For this reason, if the strength of the gradient magnetic field is made equal to that of the existing one, the limit of the correction application is as far as the degradation of the degree of uniformity of a static magnetic field which is worse by about one figure at the most as compared with the existing one. While if the strength of the field of the gradient magnetic field is strengthened, then the correction application can be applied to the magnet as well having the larger nonuniformity of a static magnetic field, if the strength of the read-out gradient magnetic field is increased, then the wider measurement band is required when receiving a signal. The S/N ratio of the measured signal is degraded in proportion to the square root of the measurement band.
The degree of uniformity of a static magnetic field in the open type magnet in a vertical direction is degraded by about four figures as compared with that of uniformity of a static magnetic field generated by the existing MRI system. As a result, in the open type MRI system using the open type magnet, it is difficult to acquire the positional information in a vertical direction. The open type MRI system using the open type magnet has the problem of improving the nonuniformity of the static magnetic field in the vertical direction.
With the method of giving the positional information by controlling dynamically the RF magnetic field shown in the prior art-3 or the prior art-4, since no gradient magnetic field is used, with respect to one-axis direction, even if the degree of uniformity of a static magnetic field is degraded by equal to or larger than two figures as compared with the existing one, it is possible to acquire the positional information. However, with respect to the remaining two-axes directions, it is necessary to give the positional information on the basis of the gradient magnetic field, and hence the distortion occurs in the positional information in the remaining two-axes directions. The open type MRI system using the open type magnet has the problem of giving accurately the positional information.